The present invention relates a novel multiple-function fluid measuring and transmitting apparatus and a measuring, controlling and transmitting system for fluids in a plant which have the function of a differential pressure transmitter which is widely used as measuring equipment for detecting a flow rate, pressure or liquid level in a chemical plant and so on as well as a control function such as a loop controller.
An example of a conventional differential transmitter is shown in FIG. 14. The pressure is delivered to be measured by differential transmitter 100 through high pressure pipe 158 and three valves 157 provided at the opposite ends of orifice 560 installed at a part of pipe 550 through which water to be measured flows.
Conventional differential transmitters are made so large and heavy for protecting a sensor disposed therein that they cannot be directly installed on a pipe 550 through which a fluid under measurement flows. Also, due to their low reliability, they are removably mounted on a special post 167 separate from a pipe or a container, so as to facilitate a regular maintenance. For this reason, such a differential pressure transmitter requires not only a pressure introducing pipe durable to a high pressure and a valve but also gas and drain releasing means 157, thereby causing a cost to be increased.
Since the detection of the flow rate requires a sensing of a temperature T and a static pressure Ps of a fluid under measurement, a temperature transmitter and a pressure transmitter are disposed on an identical pipe line to measure these two parameters and transmit them to an upper level control unit which in turn calculates the flow rate from the measured parameters. In other words, two or more transmitters respectively for sensing a pressure, differential pressure and temperature are used to calculate the mass flow rate from measured parameters as shown in U.S. Pat. No. 4,562,744.
Recently commercialized intelligent differential pressure transmitters are also used in accordance with this convention, so that even if they are provided with a composite sensor constituted of differential pressure, static pressure and temperature sensors formed on a single substrate, the temperature of a fluid under measurement cannot be correctly measured because the fluid is cooled by a pressure introducing pipe, whereby the temperature sensor is presently used for merely correcting the characteristic of the differential pressure transmitter.
For detecting the flow rate, therefore, a temperature transmitter and a pressure transmitter both are disposed on an identical pipe line, as mentioned above, for sensing a temperature T and a static pressure Ps of a fluid under measurement and transmit them to an upper level control unit which performs a calculation to measure a flow rate. For this reason, this method is problematic in that the measurement and control of a rapidly fluctuating mass flow rate is particularly difficult, and an additional cost is needed as there is required two or more transmitters.
Further, the above-mentioned conventional differential pressure transmitter is so large and heavy that it cannot be directly installed on a pipe through which a fluid under measurement flows and therefore is separately mounted on a special post apart from a pipe or a container so as to facilitate removal of the differential pressure transmitter since the reliability is so low that a regular inspection is necessary. This installation requires not only a pressure introducing pipe durable to a high pressure and a valve, but also gas and drain releasing means, thereby resulting in increasing a cost.